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Cretaceous Extinction comments (long)
Sorry it has taken me so long to collect my thoughts, but I've been
reading up on some references, looking at everyone's replies, gone back
to the archives and various sources, and I'd like to answer (and ask)
Betty's Cunningham's pineal idea is interesting, but how does it explain
the shark, horseshoe crab, and other marine critters that do not have
pineals? These animals also need ways to regulate and synchronize their
breeding and other functions. For example, horseshoe crabs seem to have
some sort of internal clock to make them break their offshore winter
torpor, so they can migrate into bays to prepare for breeding. Then
environmental cues (rising water temperatures first, full moon of May and
June next) synchronize them to breed. An impact should have messed this
up, no? Yet they made it through fine.
Related to this, I've been trying to find out if snakes, crocodilians and
armadillos have pineals or lack them. Did anyone find out for sure? All
I can add for sure is that the crocs and snakes do not have the parietal
"eye" on top, but that doesn't mean they don't have a pineal or that it
is useless. Birds have pineals, but the function(s) of this gland
is(are) still obscure. Anyone have any references?
Large animals tend to need more food than small animals, and thus may be
more prone to extinction during any environmental perturbation. Jeff
Hecht mentioned that there are more individuals in small animal species
than in large animal species; I'm guessing due to the above effect. Thus
Rob Meyerson suggests that if animals awaken to a post-impact world full
of food restrictions, more of the small-bodied individuals will make it
through to sustain that species' reproductive diversity and viability.
That explains why dinosaurs were more prone to extinction, perhaps, but
not why they lasted over 140 million years and then this particular time
(K-T) they didn't make it.
Jennifer Auld and Dave Pelley put forth a "mad photo period extinction
theory" which is interesting, but I see one exception. Crocodilians (at
least American Alligators) do hibernate (mainly in the northern end of
their range, like North Carolina). They made a good point in saying that
those subscribing to the "isolated refugia" argument must explain why a
few dinosaurs weren't also caught in the refugia and survived. That's
why I can't fathom (excuse the pun) why a bottom dweller like the
horseshoe crab made it through when other bottom dwellers didn't
(rudists, most brachiopods, etc.). If the bottom of the ocean was a
refuge for the crab, why didn't a few mosasaurs (and the other sea
monsters and ammonites) make it too?
Back to the Auld and Penney "photo period" theory: You wrote
"Physiological dependency upon seasonal changes in light levels is the
one common link that dinosaurs, angiosperms, corals, sponges, bryozoans,
etc. have in common, and it was this one factor that was the primary
cause of the Cretaceous mass extinction." If this is true, why are the
dinosaurs the only taxon in that sentence that didn't make it across,
while some representatives of all the others did? And what of all the
other critters with dependency on seasonal changes in light level that
made it too (birds especially)? This can't be the only answer.
Michael Teuton raises lots of important questions about the impact's
effects in the oceans. I too haven't seen any numbers crunched as to the
effect of this asteroid's immediate impact on ocean temperature, acidity
or chemistry. Could the chemistry of the ocean been so changed it
affected the ability of marine animals to either a) build their
exoskeletons/skeletons, b) have their remains preserved, c) obtain
untainted food or d) breathe? How would the amount of dissolved oxygen in
the ocean be affected by the impact?
And if the ocean became acidic due to acid rain fallout, so would the
fresh water on land, and what about all the freshwater animals (like
amphibians) that made it through?
Grant Harding's idea that "all sea monsters and ammonites are dependent
on only one or two types of plankton" doesn't seem likely. Extant
plankton feeders are not selective -- they just sweep it all in and eat
it without sorting. So, if the total amount of plankton falls, this lack
ripples up the food chain and less animals are supported. But if
something devastated just over 50% of the plankton species, why wouldn't
the algae and the other plankton just reproduce and take up the "slack?"
Other folks pointed out that many animals with planktonic larva did not
make it across the K-T. Then again, if the ocean was that hostile to
plankton, why did the horseshoe crab's planktonic larvae make it? And
the few brachiopods, too? The fact that about half of the plankton
species died, and the other half did not, is pivotal. If we can match
an idea that explains the dinosaur's demise PLUS the marine plankton's
selective extinctions, I think we'll have it. We don't yet!
Also, according to John Terres' Encyclopedia of North American Birds,
torpor is a state of reduced metabolism, heartbeats and temperature, with
breathing movements few and irregular, and a reduced response to outside
stimulation. This is _almost_ equivalent to the hibernation of mammals,
except that birds enter the state rapidly and uninterruptedly. To the
list of birds already posted capable of doing this John Terres adds
these: a state of _temporary_ torpor has been induced in owls, swallows,
titmice. Several birds just reduce their body temperatures: several
kinds of nightjar, Inca dove, smooth-billed ani, roadrunner, and turkey
vulture; captives of the latter did so nightly. Being this is a wide
range of unrelated bird species, and bird sizes, a hibernating dinosaur
is not out of the question, though I agree with Darren Nash that we
should be cautious to extend that far. I would think hibernation would
be useful only up to a certain body size. It would work best if the
dinosaur had some type of insulation, whether provided by nature (i.e., a
burrow or hollow tree) or feathers or modified scales.
Stanley Friesen suggests that most birds do not hibernate because they
CAN migrate; it is easier and more convenient for them to do so. My
thought is that once birds adapted to the rigors of flight (developing
great skeletal and respiratory changes, high body temperature and
metabolism) this restricted what alternatives they had available to them
to survive an environmental change. Just to maintain itself, a bird has
a minimum energy budget, which is higher than that of just about any
critter alive (short of a shrew!). Even when they lower their
metabolism it is still high in relation to cold-blooded animals and many
mammals, and they MUST meet this higher minimum or they will perish.
Migration, as dangerous and rigorous as it is, may still work better than
hibernation, since a bird can keep eating on the way up and back.
Now, a comment on John Bois' "Stealthy Egg hypothesis." Have eggs and
nests of pterosaurs been found? Can I presume _they_ were stealthy egg
layers (cliff or tree nesters? any data?)? If so, they didn't make it
through the K-T and are an exception to your idea. And dinosaurs lived
next to many egg predators from many taxons for the whole Mesozoic, so
being non-stealthy, if they were, would 've annihilated them long before
the K-T. How stealthy can a several ton animal get, any way? And we
don't have eggs and nests for every dinosaur. That's either selective
preservation or perhaps some dinosaurs retained eggs in their bodies and
hatched fewer young at a time. For now there is no evidence of this, but
if Ichthyosaurs could do this, the potential is there, and that would
make some dinosaurs _very_ stealthy egg layers!
The Cretaceous mass extinction was VERY selective. Maybe all our
theories are correct, but not for every species or taxon. I guess all
this discussion leads to one conclusion: there is no ONE cause for (or
theory to explain) all the K-T extinctions!
Now, I feel I need to vent here, being the person who started all this
recent extinction talk. I thought some of the attacks made on John Bois
were very nasty and unprofessional. Disagreeing is fine, but if you want
a subject to be dropped, just say so. You don't have to accuse someone
of being dense or blind to evidence or whatever. From the threads of
this discussion I saved, I don't see anything that John said or asked
that should have provoked this attack. We can all make up our own minds
as to the validity of a theory based on the evidence and arguments
presented. The Truth will out. And though we may want to restrict
discussion to a certain range of ideas, I don't like the chilling effect
of blasting someone for exploring their ideas in an enthusiastic,
curious, persistent manner. These are all good scientific qualities.
The whole point of subscribing to this list is to get information about
dinosaurs. Asking questions is the only way to learn. Ideally, every
one should feel welcome to submit questions, references and information
freely. So, let's please stop these _ad hominem_ comments. That's not
science. Science is attacking the argument, not the person. _ Ad
hominem_ arguments only inhibit folks from even asking a question or
joining the discussion. What data could we be missing because of this
intimidation? We'll never know.
Since the recent consensus seems to be that continuing this discussion is
pointless, feel free to respond to me off list. Thanks to all for your
information, critiques, questions, and patience. I for one have learned
Education Associate, Virginia Living Museum
All questions are valid; all answers are tentative.